Most people familiar with this section of the forum are very familiar with how fluorination can be used to selectively strip uranium out of a fluoride salt mixture by converting UF4 to UF6 and how aggressive flame fluorination like FREGAT can be used to convert spent nuclear fuel to fluoride form producing UF6 in the process.

Similarly we know that chlorination can digest spent nuclear fuel and convert it to a chloride form quite effectively.

My question is this, can low temperature chlorination be used to selectively strip UCl3 from a chloride melt in the same way that low temperature fluorination strips UF4 from a fluoride melt? And will that higher UClx form be primarily UCl6 or UCl5.

The overall objective of my question is to determine if low temperature chlorination is potentially an effective means of stripping bred 233UCl3 out of a NaCl/ThCl4 melt.

We must have low volatility for the form of uranium in the reactor. We don't want even a modest percentage of it to form a gas - bad things happen then. Good thing then that UF3, UF4, UCl3 (and I presume UCl4) have low vapor pressures (in other words no at this point they won't come out of the salt).

Could we add chlorine gas and get the uranium to come out that way? I would think so. Surprisingly though that is not the plan for IFR.

Note that F2 gas is expensive so a better plan is to first dissolve the LWR/SNF into HF acid to create UF4 and then promote UF4 to UF6 using F2. ORNL bubbled the F2 through the salt when then removed the 235U in preparation for running with 233U. The F2 was hard on the vessel and put a lot of corrosion products into the fuel salt that messed up results for a while until it got cleaned out. They were working on a cold salt lining to protect the vessel for future. They also ran experiments with hot (600C) F2 gas and did a very good job at removing plutonium but for some reason concluded that this was not a viable way forward. I've wondered if we add NF3 to the fuel salt if the uranium would steal the F3 away and we would get UF6 gas and N2 gas while avoiding the pain of dealing with hot F2.

Uranium easily goes to hexavalent state and can be removed from a mixture. UCl6 is volatile and could be extracted as vapour. Fluorine can easily convert uranium as well as plutonium to volatile hexafluorides. There are differences in the volatility of chlorides and fluorides of other elements too.
Chlorine has two major isotopes, one of which, Cl35, nearly three-fourths, absorbs too many neutrons to produce problem products. Natural chlorine may be good for pyroprocessing through chloride volatility but Cl37 will be required for liquid salt reactors.
I think that the IFR proposal includes removing most of uranium as hexachloride https://docs.google.com/viewer?url=http ... b29596.pdf
before electrorefining to get the fuel for fast reactors.

The IFR plan is to use electrorefining to separate the uranium.
It is actually fairly difficult to promote PuF3 up to PuF6 - ORNL did accomplish it (see Falling Drops in the repository) but it took creating small droplets (around 50u IIRC) and high temperatures (around 600C).

The recent ORNL proposal for using a fast spectrum reactor to burn off TRUs did propose using chlorine volatility.

Thanks Lars, I was hoping to use the reluctance of Pu to move as an advantage and actually encourage that effect by deliberately chlorinating it at low temperatures in an attempt to leave as much Pu behind as possible. Specifically, if we have a NaCl/PuCl3/ThCl4 melt where the Pu is fuel which is externally added, the thorium is the fertile and U233 is extracted by chlorination and is the by-product that we are targeting, it would be quite desirable to be able to pull off the bred U233 with minimal Pu content.

My thinking is this: Pu on its own is troublesome on a number of fronts and without wanting to relitigate the relative merits and non-proliferative nature of well burned reactor grade Pu, it seems to me that if one simply had a system where Pu only ever goes into a running core and is never extracted, then that would be a good thing. The ability to burn Pu and effectively convert it to denatured U233 would be a strong net positive from a non-proliferation point of view.

Vacuum distillation may also be useful. At the temperatures and vacuum that ORNL operated the fluoride vacuum still experiment, which worked very well, NaCl, ThCl4 and UCl4 all come out in the product stream.

Thanks Cyril, that's the other spanner in the toolbox, I figure that distillation could work well for FP removal, the only thing there is that UCl3 and PuCl3 have similar vapour pressures so it may be hard to get one without the significant contamination of the other by distillation alone. My hope is that with chlorination at the right temperature, that only the the U will mobilise, leaving the Pu behind in the melt. That's the general idea, how close it is to reality, I don't know, hence the post.

Lindsay, I'm a little confused with what you are trying to accomplish.

Here is what I'm thinking:
For production reactors are fluoride based and use fluorination to pull out the uranium without the plutonium - though if a little bit of Pu that comes with it no harm is done. Distillation is used to recycle the salt. Plutonium and fission products go together to the stream going to the centralized cleanup.

For centralized cleanup reactors chloride makes sense but then we want the plutonium to go back into that reactor so it is actually good if the uranium and plutonium flow together.

Cyril R wrote:Well, you could probably get the UCl3 to become UCl4 by adding some chlorine - PuCl3 doesn't like to go up to PuCl4 very much, but UCl3 does readily become UCl4.

Still there could be surprises. Chlorides have more oxidation states than fluorides.

Yes, that is one of the challenges with chlorides that make them that little less predictable than fluorides as I understand it. Hopefully that's something that the chemists can manipulate to our advantage, but it could just as easily be a pain in the tail.

Lars wrote:Lindsay, I'm a little confused with what you are trying to accomplish.

Here is what I'm thinking:
For production reactors are fluoride based and use fluorination to pull out the uranium without the plutonium - though if a little bit of Pu that comes with it no harm is done. Distillation is used to recycle the salt. Plutonium and fission products go together to the stream going to the centralized cleanup.

For centralized cleanup reactors chloride makes sense but then we want the plutonium to go back into that reactor so it is actually good if the uranium and plutonium flow together.

What do you have in mind for a chloride machine?

For deployment in a fully secured environment, perhaps at a central fuel processing facility. At a high level I want a black box into which you add Pu, Th and some recovered U or DU, while removing FP's, electricity and denatured U233. By magic or nuclear reactions, one is destroying Pu while breeding denatured fissile.

The desired outcome is a machine into which we tip Pu, but never extract it on its own. For the Pu it is a one way trip, remaining in fuel salt until burned. Thorium is the fertile material and chlorination is the means by which one extracts bred U233 that is Pu free or close to it. For FP extraction, low pressure distillation can be used to target lighter FP's, then heavier FP's all without Pu separation.

The more bullish on this forum will say that this is pandering and semantics, maybe it is, but if it can be done, if you can demonstrate that Pu is never extracted from fuel salt and remains in core until consumed, I'd buy that. And just to add another philosophical point, most people with a basic understanding of nuclear materials understand what denatured uranium is and that is broadly accepted, so if we can do that, then we should. The exact potential for reactor grade Pu in a weapon seems unclear to me and many others, so I don't think that we are going to win any PR battles by sticking to the mantra that reactor grade Pu is unsuitable for making into a fission weapon. Sorry off topic a bit, but that's the thinking behind wanting to minimise the role of and the extraction of Pu.

Lindsay wrote:
For deployment in a fully secured environment, perhaps at a central fuel processing facility. At a high level I want a black box into which you add Pu, Th and some recovered U or DU, while removing FP's, electricity and denatured U233. By magic or nuclear reactions, one is destroying Pu while breeding denatured fissile.

The desired outcome is a machine into which we tip Pu, but never extract it on its own. For the Pu it is a one way trip, remaining in fuel salt until burned. Thorium is the fertile material and chlorination is the means by which one extracts bred U233 that is Pu free or close to it. For FP extraction, low pressure distillation can be used to target lighter FP's, then heavier FP's all without Pu separation.

The more bullish on this forum will say that this is pandering and semantics, maybe it is, but if it can be done, if you can demonstrate that Pu is never extracted from fuel salt and remains in core until consumed, I'd buy that. And just to add another philosophical point, most people with a basic understanding of nuclear materials understand what denatured uranium is and that is broadly accepted, so if we can do that, then we should. The exact potential for reactor grade Pu in a weapon seems unclear to me and many others, so I don't think that we are going to win any PR battles by sticking to the mantra that reactor grade Pu is unsuitable for making into a fission weapon. Sorry off topic a bit, but that's the thinking behind wanting to minimize the role of and the extraction of Pu.

I'm still confused. You stated it was for a secure facility - if so what is the issue with crummy plutonium? How did the plutonium get to the reactor anyway? Once inside the core it will degrade steadily so it is always less of a proliferation risk than when it went in. What is the denatured 233U for? If you have such a machine and the nation decides to withdraw from the non-proliferation treaty it wouldn't take much tuning to convert it to a breeder that outputs 233U (with a pretty high 232U content since it is a fast reactor). That aside.
I guess you could run a machine with almost all fertile being thorium and add just a enough DU to denature the 233U when you take it out. You put Pu in and wait a long time before you pull it out since you can tolerate 5-10 years worth of fission product build up. That should let the Pu degrade significantly in quality before it comes out of the reactor.

Lars wrote:I'm still confused. You stated it was for a secure facility - if so what is the issue with crummy plutonium? How did the plutonium get to the reactor anyway? Once inside the core it will degrade steadily so it is always less of a proliferation risk than when it went in. What is the denatured 233U for? If you have such a machine and the nation decides to withdraw from the non-proliferation treaty it wouldn't take much tuning to convert it to a breeder that outputs 233U (with a pretty high 232U content since it is a fast reactor). That aside.
I guess you could run a machine with almost all fertile being thorium and add just a enough DU to denature the 233U when you take it out. You put Pu in and wait a long time before you pull it out since you can tolerate 5-10 years worth of fission product build up. That should let the Pu degrade significantly in quality before it comes out of the reactor.

It is a weak argument so I won't try to win it, but the affirmative goes something like this...

If society at large is concerned about increasing stockpiles of Pu of any grade, it would be nice to offer them a system where one effectively swaps Pu for denatured U233. While some transportation of Pu or Pu bearing materials is required it is limited to one way trip, the one that takes it into the reactor core, to never emerge. Security is required because it is a strong breeder and could breed Pu more easily than U233, or could breed relatively pure U233 but at least it would be well laced with U232, as you mention. So I'm very sympathetic to your comment regarding a nation state having one of these, withdrawing from the NPT and subverting the reactor to produce materials for weapons, that's entirely possible.

It could also make a nice option for dealing with spent LWR MOx, as I understand that recycling of spent MOx into more MOx is problematic in a couple of different ways. Destruction of weapons grade Pu is another possible function that would be a good marketing point. There would be enough flexibility within the salt mix to permit limited quantities of PuCl3 to be added on demand so long as they are balanced by matching additions of ThCl4 to act as a burnable poison.

As for U233, I'd be interested in deploying that where it could do the most good, the first thought is to convert to fluoride form and use as DMSR fuel improving the fuel cycle over what can be achieved with U235 (a small but potentially useful improvement).

So in summary, a bit OTT for the rational amongst us who are not so concerned about reactor grade Pu, but may have some selling points for those keen to minimise Pu stockpiles of all kinds. Allowing for the startup inventory requirements, a design like this could soak up a significant chunk of the existing Pu inventory and put it beyond use more or less immediately, sequestered in an operating core until destroyed. That's the way the marketing pitch would describe it.

Lars wrote:Does anyone know how UF4 is converted to UF6 for standard enrichment? What is added to donate the fluorine and at what temperature?

I just googled this document, about the various processes and apparatus used in uranium conversion/deconversions, which clearly states that flame fluorination with elemental fluorine is employed:

The reaction of uranium tetrafluoride (UF4) with
fluorine to form UF6 is typically performed in
vertical tower reactors. Uranium tetrafluoride
powder and preheated (325 to 375°C) fluorine
gas (F2) are fed into the top of the reactor. An
excess of F2 is used to ensure complete
chemical reaction, which is rapid and
exothermic. Temperatures in the flame where
UF4 and F2 react can reach 1100°C, although
the walls of the reactor are typically maintained
between 325 and 540°C. Unreacted UF4 is
collected in an ash receiver positioned at the
bottom of the tower reactor. A cooled cyclone
separator may be used to provide both gas
cooling and collection of larger UF4 particles.
After the cyclone, filters can be used to remove
small UF4 particles.
Offgas is sent to a second, smaller stage, which
uses excess UF4 to ensure that all of the fluorine
is consumed. Unreacted UF4 and other solid
uranium fluorides are recycled to the UF4 feed
hopper.
The UF6 product condenses as a white powder
resembling snow and is collected in a batchoperated
heat exchanger (cooler) that is
weighed continually as the “snow” accumulates.
When its weight reaches a predetermined limit,
the heat exchanger is taken off line to be
emptied of its UF6 contents, and another heat
exchanger is placed on line to be filled. The UF6
“snow” is liquefied by warming and piped into
product collection cylinders.

Typical Appearance
As Manufactured: A typical vertical tower
reactor is a tube about 20 cm in diameter by 4 m
tall (Figure 41.2). The tower is fabricated from a
pipe made of a material such as MonelÒ, which
can withstand the hot fluorine environment.
Integral cooling coils running the length of the
reactor may be used to control wall
temperatures.